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The user selects a date in a Date field. You can save the corresponding day of the week (or other aspects of the date) in a Hidden or text field. This Hidden or text field can be used in Conditionals, among other things.

If you want to save the month or some other aspect of the date, change the format from ‘l’ to the format you prefer. You can see the other format options here.

Change 847 to the id of the Hidden or text field to hold the day of the week and 846 to the Date field.